Treatment of glassine paper



Dec. 5, 1933. v R Q CHARRON ET AL 1,938,133

TREATMENT OF GLASSINE PAPER Filed Oct. 27, 1927 Patented Dec. 5, 1933 UNITED STATES PATENT OFFICE OF GLASSINE PAPER Roy 0. Charron and Willard E. Swift, Worcester, Mass., assignors to United States Envelope Com- Dany, Springfield, Mass, a corporation of Maine The present invention relates to a process of treatment for so-called glassine paper, which is a highly transparent material that is extensively used for the panels of window envelopes, and also as a transparent covering for fancy boxes and the like.

The invention has as its principal objects to improve the texture and to increase the trans-- parency of such material, while at the same time retaining in the paper its inherent physical properties, i. e., toughness, strength, and ability to bend without cracking,-which distinguish it from the gelatinous transparent covering materials now in ordinary use.

As is well known, glassine paper is paper in which a high degree of hydration of the cellulose fiber has been effected, usually as the result of prolonged and severe beating and refinement of the pulp. Such hydration and comminution produce partial coalescence of the cell walls, thus forming a more homogeneous and membranous-like sheet than ordinary paper, and possessing a structure that acquires, under heavy calendering, a high degree of transparency. However, the product, glassine paper, is inherently brittle, horny, and inelastic-and these are properties which operate as limitations on its use, particularly as a transparent wrapping material.

We have discovered that by subjecting the glassine paper to certain treatment, its inherent brittleness can be eliminated,.and its toughness and elasticity measurably increased. This treatment, however, involves the loss of some of its transparency, and to remedy tins, we have devised a further treatment which restores, or even increases, the original transparency, while at the same time preserving the softened and improved texture which is produced by the initial treatment. As a result of the successive treatments, 2, product is obtained which is superior in every way, and particularly in' texture and transparency, to ordinary commercial glassine paper.

The drawing shows a diagrammatic sketch of the apparatus used in carrying out the invention.

According to the invention, the initial softening treatment involves merely a brief immersion of the glassine paper in a suitable liquid of characteristic 0-H radical composition, for example, any polyhydric alcohol, such as glycerine, which, to promote the softening action, may be heated. Preferably, as shown in the accompanying drawing, the glassine paper is in the form of a continuous web 1, being taken from a supply roll 2, and fed beneath suitable rollers 3, 3 which are immersed in a tank or pan 4 of the initial treating liquid.

We have found that glycerine, preferably sufficiently heated to drive off water which it absorbs from the atmosphere, may very advantageously be used as the softening bath in the tank or pan 4, but there are a number of other liquids of the same characteristic 0-H radical content that are capable of producing practically as good effects when used for this initial softening bath, among them, other poly-hydric alcohols, also ethylene glycol, hexane, oleic acid, hydrogenated oils, and castor oil. Consequently the invention, as regards this initial softening treatment, is not to be limited to any particular liquid, such as glycerine, except as specified in the appended claims.

Leaving the tank 4, which contains the bath of softening liquid, the web of paper 1 is passed between suitable devices, such as rotary brushes 5, 5, which act to remove the excess softening liquid from both surfaces of the web, after which said web is drawn through an elongated heating chamber 6 by means of suitable feed rolls 7, 7, which are disposed preferably at the far end of said chamber. The chamber 6 is of sufiicient length, and the heat applied therein is of suflicient intensity, so that when the paper reaches the feed rolls '7, 7, it is in a thoroughly dried condition, with all surface liquid entirely evaporated and driven off. The paper thus treated, though soft and pliable, as compared to commercial glassine paper, is nevertheless of a-somewhat impaired transparency, and its further treatment in the second part of the process is for the purpose of restoring, and even increasing, the original transparency, as will now be described.

We have discovered that such transparency can be very measurably and appreciably increased by applying to one side, or to both sides, of the paper a liquid coating carrying nitro-. cellulose in solution,--heat being preferably applied to drive offthe volatile ingredients of the solvent, so as to leavea surface deposit or coatess, the treating liquid is-prepared by dissolving nitro-cellulose in a suitable substance, or mixture of substances, whose boiling point is relatively low, so. as to promote evaporation when the solution is spread in a thin film on the surface of the web. Such a solution of appropriate viscosity may be obtained by dissolving any of the several nitrated cottons in any of the well known solvents for nitro-cellulose, such as acetone, amyl acetate, ethyl or methyl benzoate, and the like, or in a suitable mixture of such solvents, or if desired, discarded photographic or motion picture films, whose composition, as is well known, is principally nitro-cellulose, may be dissolved in such solvents, to provide the liquid for the transparentizing treatment.

It is to be understood that the invention is not in any sense limited to the source of nitrocellulose, nor to the solvent used to hold it in solution, it being obvious that solvents having higher boiling points than those mentioned above, for instance, benzyl-benzoate, diphenyl ether, and the like, may, under some circumstances, be advantageously employed in order to raise the refractive index of the solution to a point substantially corresponding to the refractive index of the glassine paper. This dissolving of the nitro-cellulose, as supplied from nitrated cottons, films, or from any other desired source, is obtainable, using solvent liquids, either with' or without the assistance of mild heating,-and we also find it preferable sometimes to add to the transparentizing solution before application to the paper to be treated, a suitable small quantity of ethyl carbonate, or ethyl lactate, or other equivalent substances, whose action to prevent the appearance of blushes or spots in the drying of the transparentizing'film, after it has been applied to the surfacing of the web.

Such application to the web 1, already softened and made, pliable by the initial treatment in bath 4, may be conveniently carried out by leading the web from the rollers 7, 7 into a tank or pan 8, containing the nitro-cellulose solution, the web passing under suitable rollers 9, 9 immersed in said tank 8. Leaving the tank 8, the web 1, wet on both sides, is fed through a second elongated heating chamber 10, by means of feed rolls 11, 11, at the far end of said chamber. Said chamber 10 is of suflicient length, and the heat applied therein is of suflicient intensity, so that when the paper reaches the feed rolls 11, 11, it is in a thoroughly dried condition, the liquids which carry the nitro-cellulose in solution having been entirely evaporated and means The deposit, practically of nitro-cellulose, and substantially of the same refractive index as the paper itself, which is left on said paper by the above described drying following immersion in tank 8, imparts to said paper a glass-like homo geneous surface, on both sides, and this is ac: complished without substantial impairment of the softness and pliability of the material, as produced by the initial treatment in the bath 4.

We claim,

1. The treatment of glassine or like paper product rendered transparent by high hydration and heavy calendering, which consists in first softening said paper to improve its texture, and then depositing on its surface a film of nitro-cellulose to counteract the impairment of its transparency by said softening.

2. The treatment of glassine or like paper product rendered transparent by high hydration and heavy calendering, which consists in first subjecting said paper to the softening action of a substance of 0-H radical content, whereby the brittleness of the paper is substantially removed, and then depositing on the surface of said paper a film of cellulose compound of substantially the face a liquid containing a cellulose compound in solution, and finally driving off the volatile constituents of said liquid, thereby to leave on the surface of said paper a deposit which overcomes the impairment of transparency produced by said softening bath.

4. The treatment of glassine or like paper product rendered transparent by high hydration and heavy calendering, which consists in subjecting said paper, to the softening action of glycerine and then-to the transparency-increasing action of a cellulose compound in solution.

5. The treatment of glassine or like paper product rendered transparent by high hydration and heavy calendering, which consists in subjecting said paper to the softening action of ethylene glycol, and then to the transparency-increasing action of a cellulose compound in solution.

ROY C. CHARRON. WIILARD E. SWIFT. 

